(a) Field of the Invention
The present invention relates to an optical recording disk and, more particularly, to an optical recording disk for recording data thereon using a laser radiation.
(b) Description of the Related Art
Recently, there is a strong demand for an optical recording medium which is capable of recording/reproducing data thereon by using a laser radiation of a smaller wavelength, which is smaller than a current 780 nm or 830 nm wavelength, for a high-density recording. Among other optical recording media capable of smaller wavelength recording, an optical recording disk using an organic pigment has advantages of its low costs and easy processing thereto.
Cyanine or similar substance is recently proposed as an organic pigment for such a smaller wavelength recording in JP-A-6(1994)-3306086, JP-A-7(1995)-161068, JP-A-7(1995)-262604, JP-A-7(1995)-125441, JP-A-7(1995)-266705 etc. In the recording disk using such a substance, it is generally considered that changes in the optical constants and reduction of film thickness due to a thermal decomposition (or pyrolysis) of the organic pigment, as well as transformation or deformation of the substrate due to melting of the substrate, are generated in the recorded section of the disk, as in the case of 780 nm wavelength recording in a CD-R disk.
In such a recording disk, wherein the modulation factor during the recording is obtained by both or either of the decomposition of the pigment and the deformation of the substrate, cross-talk is a major problem since the transformation in the recorded section is large, and it is especially serious if a large bit extends toward the land area from the adjacent groove area in the case of groove recording.
In the case of a smaller track pitch to be applied to a digital audio disk (DVD), the cross-talk problem is more serious. In the above-mentioned patent publications and other publications such as JP-A-4(1982)-109441, JP-A-4(1992)-1182944, JP-A-6(1994)-282873 and JP-A-6(1994)-28287, although CD-R disk having a high reflectance and excellent characteristics are described, the problems and solutions thereof inherently involved in the organic pigment used in the high-density recording medium are not taught nor suggested.
To solve the above problems, the present inventors showed in Japanese Patent Application No. 7(1995)-213501 that small area of the chemical changes in the recorded section, sufficient modulation factor, high reflectance and large signal amplitude can be obtained by employment of the pigments having a characteristic of steep and large reduction in weight as a principal ingredient or an additive of the recording materials, together with the skeletons which have such a characteristic.
However, the present inventors found that the cross-talk problem arises again in the case of a smaller track pitch and a smaller groove width, such as in a recording disk having a high recording capacity of 5 giga-byte (Gbyte) or more recorded on a single surface.